Uninterruptible power supply systems which provide truly uninterrupted power to the load on the failure of the main or working supply are often used to supply load power to critical loads, 1. e., a load for which it is crucial to have a continuous supply of power without interruption or disturbance in power. This type of UPS is widely used in broadband communication systems, e.g. telecommunication and CATV systems involving the transmission of data. This type of UPS is often referred to as a "true" UPS system to distinguish the system from standby systems in which an alternate power source must be connected into, i.e. switched into, the system upon failure of another power supply. The need to switch an alternate power supply into the system causes a disturbance or interruption of the power flow to the load over the interval it takes to detect the failure and switch the alternate power source into the system.
A true UPS system may be based on several different topologies. In a conventional dual conversion topology, well known in the art, an inverter converts d.c. power to a.c. power and supplies it to the load on a continuous basis. This type of system most frequently has a first power supply, generally an a.c. utility power line, which is rectified to provide a d.c. power source which in turn provides the d.c. power required by the inverter The rectifier for the a.c. power supply will also maintain the charge on batteries of a battery power source for supplying back-up d.c. power for the inverter. Upon failure of the a.c. power supply, the inverter continues to be supplied with d.c. power, without any interruption, by the battery power source which serves as the second, or alternate, source of power for the inverter. Upon restoration of the a.c. power supply, the rectifier once again powers the inverter and, at the same time, recharges the batteries. Because power is always supplied from either the a.c. power or the battery power supply there is no disturbance or interruption of the a.c. output of the inverter. In a normal operation, with both power sources available, the first power source is given priority over the alternate power source by virtue of the fact that its d.c. power source has a higher effective voltage than that of the alternate battery power source. Even though both sources are available, all power is supplied by the first power source until the first source fails.
U.S. patent application No. 09/075,739 filed May 11, 1998 by Howard H. Bobry (the inventor herein) describes an alternative true UPS topology offering improved efficiency through the use of a plurality of inverters. As in the conventional dual conversion topology, priority of the power sources is determined by relative voltage levels, but in the Bobry topology priority is determined by the relationship of effective magnitudes of a.c., rather than d.c., voltages. In this description, the "effective voltage" of a power source will mean the a.c. or d.c. voltage, the magnitude of which determines its priority level for supplying supplies power for the inverter of the UPS.
In any UPS application having a battery power source, or any back-up power source whose condition may deteriorate, it is desirable to test the batteries periodically to assure that they are capable of functioning as a reliable power source in the event of a failure of the first power supply (generally a rectifier powered from an a.c. power line). Typically this test is carried out by disconnecting the first power supply from the input of the UPS, e.g., disconnecting the a.c. power lines from a utility power supply. The UPS then operates from the battery power source as it would during a real power failure. This procedure amounts to creating an artificial power failure in order to test the ability of the battery power source to provide power. While this test method is effective in that it permits an evaluation of the batteries, it carries a substantial risk of interrupting power to the load. A number of battery problems, such as loose, corroded, or open connections, or a failed or failing battery cell, can prevent the batteries from supplying the necessary power, i.e., the very act of testing the batteries to assure that power will be uninterrupted may actually result in an interruption of power with no operating back up power supply.
It is an object of the present invention to provide a method of safely testing the battery or batteries of an alternate battery power source of a UPS, and apparatus therefor, the method and apparatus being such that the battery power source is called upon to supply power as it would upon a failure of another power source, or sources, with the other power source or sources being maintained as backup so that there is no interruption of load power, caused by switching or otherwise and regardless of any failure of the batteries, even if the batteries fail abruptly and totally.